Probing Atomic Scale Structure and Catalytic Properties of Cobalt Oxide Model Catalysts

Abstract

Cobalt oxides are known to be active catalysts for a number of chemical reactions, but very little is known about the atomic scale processes
responsible for the activity. The research presented in this thesis is focused on obtaining an atomic scale understanding of the chemistry of wellcharacterized
cobalt oxide model catalyst surfaces consisting of pristine and defective CoO and Co3O4 thin films with the (111) and (100)
terminations supported by Ag(100), Ir(100), and Au(111) single crystal surfaces. The structure and the adsorption properties of probe molecules
onto these cobalt oxide model catalyst surfaces are studied under ultra-high vacuum conditions using the interplay of X-ray photoemission
spectroscopy (XPS), scanning tunneling microscopy (STM), and low energy electron diffraction (LEED). Further, high pressure XPS (HPXPS)
is used to study the stability and phase transitions of the cobalt oxide model catalysts in more realistic gas environments. As a side project to
the work on cobalt oxide thin films the thesis gives a comprehensive spectroscopic picture of Ir(100) surface reconstructions and molecular
adsorption onto these surfaces.
The adsorption experiments of H2, CO, CO2, and H2O probe molecules give a detailed picture of the surface chemistry of Co oxide surfaces and
it is demonstrated that Co ions naturally found on the surface of Co3O4(111) and Co3O4(100) thin films or artificially created on the CoO(111)
surface are extremely important for chemical properties of the surface. Water dissociation, carbonate formation, weak adsorption of CO and
CO2 are examples of processes that only take place in the presence of Co surface ions. The work at more realistic gas pressures in the mbar
regime demonstrates that Co oxide thin films should be seen as dynamic films that easily change phase between the CoO and Co3O4 structure
in response to the gas composition.
To summarize, the work presented in this thesis is important for the fundamental understanding of cobalt oxide surfaces and their catalytic
properties, and hopefully, this fundamental understanding can be used to develop new and better cobalt oxide based catalysts.

Abstract (Swedish)

Catalysts are used to produce a large fraction of the materials we use in our modern society. A
very famous example is the highly efficient catalysts that are used to fix nitrogen from the air into
artificial fertilizer salts. Without this catalytic process, it is difficult to imagine that we could feed
the current population of earth. Artificial fertilizers are, however, not the only product that uses a
catalyst for its production. In fact, almost all products produced in the chemical industry such as
plastic materials, paints, coating materials, gasoline, drugs, etc. use catalysts for their production.
Catalysts are also used extensively for cleaning of exhaust gas from power plants, trucks, and cars.
As an example, the catalyst in a car convert carbon monoxide gas (CO) to non-toxic carbon dioxide
(CO2). Unfortunately, the catalyst in the car is built partly from very expensive metals such as
platinum and palladium.
As discussed above catalysts are used extensively both for the production of modern materials and
for reducing the amount of toxic chemicals we release into our environment. Most of the catalyst
materials we use today have been found by trial and error methods and knowledge of why and how
the chemical process take place on the catalyst material is therefore often very limited or missing
fully.
The goal of the present work has been to improve our understanding of chemical processes taking
place on cobalt oxide based catalysts. Instead of studying real and complex cobalt oxide catalyst
materials we have studied thin and highly idealized cobalt oxide films. Using these highly idealized
model systems of the real catalysts we studied chemical processes at the atomic scale level. One
important take home message of the studies is that single cobalt atoms found on the surface are
essential for the function of the catalysts surface and in particular for how it interact with gas
molecules.
Hopefully, the present fundamental work on cobalt oxide catalysts can be used to develop new and
better catalysts of this material. Furthermore, the work adds knowledge to our general
understanding of metal oxide films and their catalytic applications.

• author: Arman, Alif
• supervisor: Jan Knudsen; Edvin Lundgren
• opponent: Prof. Dr. Over, Herbert, Justus-Liebig-Universität-Giessen, Germany
• organization: Synchrotron Radiation Research
• publishing date: 2016-12
• type: Thesis
• publication status: published
• subject: Natural Sciences
• keywords: catalysis, model systems, Co3O4(111), Co3O4(100), CoO(111), X-ray photoelectron spectroscopy, Scanning tunneling microscopy, Fysicumarkivet A:2017:Arman
• pages: 203 pages
• defense location: Rydberg lecture hall, Department of Physics, Sölvegatan 14A, Lund
• defense date: 2017-01-27 13:00:00
• ISBN: 978-91-7753-141-8
• ISBN: 978-91-7753-140-1
• language: English
• LU publication?: yes
• id: f1435420-2eb2-42e5-a0d9-b52e34f7a6e7
• date added to LUP: 2017-01-13 15:58:16
• date last changed: 2018-11-21 21:28:55